1. Field of the Invention
The present invention relates to a high-resolution wide field adaptive optics imaging system based on a compact multi-reference wavefront sensor.
The shortcomings of single reference systems in the severely restricted field of corrected view is solved with multi-conjugate adaptive optics (MCAO) systems; however this is at high cost and gives bulky instruments, especially problematic in ophthalmology. Another problem, especially in the study of the human eye, is unwanted light from parasitic source reflections and light from unwanted object regions.
2. Description of the Related Art
The main purpose of an adaptive optics (AO) system is to correct for dynamical optical aberrations that blur the image of a normally extended object under investigation. Typical examples of such images are ground based telescope images blurred by the atmosphere and retinal images blurred by the eye's optical aberrations. Related to a perfect image point there will be a spherical wavefront converging towards the point. A wavefront is the locus (a line or surface in an electromagnetic wave) of points having the same phase. When the image point is blurred by aberrations, the corresponding wavefront will deviate from the perfect one, and the difference between the two wavefronts is called the wavefront aberration. Conventional AO systems comprise a wavefront sensing arm measuring the wavefront and a science arm used for imaging and other measurement purposes. A so-called Shack-Hartmann wavefront sensor in the wavefront sensing arm usually makes use of a point source, located somewhere within the image to be corrected, to estimate the wavefront. The aberrated wavefront associated with this point source is measured by subdividing it into small regions subjected only to local tilt. From each of these small regions, an image of the reference source is formed. These images will be translated according to the local wavefront tilt, which can then be measured. The collection of point images is called a Hartmann pattern. A wavefront modulating device is inserted in the part of the optical path common to both the wavefront sensing arm and the imaging arm. This can be either a deformable mirror or a reflective or transmitting phase modulator. Hence the sensor works in a closed loop where the original aberrations are cancelled by the wavefront modulating device. Conventional AO suffers from the drawback that if the image aberrations vary over the image, a single wavefront modulator will correct only a small region around the location of the reference source. Having more than one modulating element can extend the corrected image region, but this also requires more than one reference source. Such a scheme is called multi-conjugate adaptive optics (MCAO).
Wavefront sensing and AO have caused a renaissance of vision research. Collaborations with astronomy research groups have been instrumental in developing AO systems for ophthalmology and vision science. Confocal scanning laser ophthalmoscopes have been available for some time for imaging the retina. The resolution achieved by these instruments is limited by aberrations in the cornea and lens of the eye. By including AO correction in a scanning laser ophthalmoscope, it is possible to correct these aberrations and so obtain diffraction-limited images of the retina. These reveal the fine structure of the eye, including individual photoreceptors, blood vessels, and nerve fibers. Early studies have revealed new information about the organization and function of the retina, changes indicating disease and the optical and neural limits of human vision.
The resolution of the eye is potentially affected by retinal resolution and neural factors as well as by optical aberrations. Studies with AO corrected instruments suggest that even sharper resolution can be obtained by correcting higher-order aberrations in the eye. Spectacles have been used for centuries to correct defocus. It is only in the last 150 years that spectacles have been used to also correct astigmatism. Adaptive optics systems offer the possibility of accurately measuring high-order aberrations in individuals and customizing the correction to each person. Methods for implementing this are currently being explored; customized contact lenses, intraocular lenses, or customized laser refractive surgery.
The ability to image individual photoreceptors in the living eye may also improve our ability to diagnose eye disease. Age-related macular degeneration (AMD) is a disorder of the macula, a small area at the very center of the retina responsible for our sharpest vision, in which damage to the photoreceptor layer causes severe vision loss in the elderly. The early stages of AMD comprise pigmentary changes and drusen development, which contrasts with the more severe forms that develop geographic. atrophy or neovascular membranes. It is this early form of AMD that is important, as it heralds the loss of a normal retinal status and indicates a greater risk to central vision loss. Glaucoma, a disease in which blindness ensues from the gradual loss of optic nerve fibers, can only be detected with conventional imaging techniques after a significant amount of damage has already occurred. More accurate measurement of the nerve fiber layer thickness around the optic nerve head will allow earlier detection of this disease. Diabetes is another disease that affects the retina by the formation of microaneurysms in the retinal vasculature. Treatment requires accurate delivery of a photocoagulating laser beam, which could benefit from a higher resolution view of the critical retinal region.
Today, no commercial AO corrected ophthalmic instruments are available. The first research instruments used low-order deformable mirrors for aberration correction, typically 37-actuator faceplate mirrors. Higher order correction systems are now in use. These typically use 97-actuator deformable faceplate mirrors that are so large that they require elaborate beam expansion optics. These systems are expensive and bulky, which makes them unsuitable for clinical applications. The push in this area is now towards Micro Electro-Mechanical Systems (MEMS) deformable mirrors that are much smaller and cheaper than piezo-electrically driven faceplate mirrors. The availability of suitable MEMS devices will revolutionize the market for clinical adaptive optics corrected ophthalmoscopes.
Adaptive correction of the eye was first attempted in 1989 by Dreher (Dreher, A. W. et al. Appl Opt, 1989. 28(4): p. 804-) who used a 13-actuator segmented mirror to correct a subject's astigmatism by applying his conventional prescription for spectacles for adjustment of the mirror form. Liang (Liang, J. et al. J Opt Soc Am A, 1997. 14(11): p. 2884-92) constructed an AO system that could correct higher order aberrations using a large 37-actuator single sheeted deformable mirror (made by Xinetics Inc). Vargas-Martin (Vargas-Martin, F. et al. J Opt Soc Am A, 1998. 15(9): p. 2552-62) evaluated the performance of a liquid crystal spatial light modulator for the correction of aberrations in the human eye. These examples all used static AO wavefront correction, e.g. optical aberrations of an eye were first measured using a wavefront sensing device, and the obtained information was subsequently applied to a deformable mirror or spatial light modulator.
Ocular aberrations, however, are dynamic. Aberrations change over time, with, for example, accommodation (the eye's lens change of focus), and with gaze angle. There is thus a need for real-time AO correction, as demonstrated by Fernandez (Fernandez, E. J. et al. Optics Letters, 2001. 26(10): p. 746-748) and Hofer (Hofer, H. et al. Optics Express, 2001. 8(11): p. 631-643). There are also dynamic processes occurring in the retina that, if they are to be studied in detail, require the implementation of real-time AO correction. The AOSLO (Roorda, A. et al. Optics Express, 2002. 10(9): p. 405-412), a confocal scanning laser ophthalmoscope with AO correction, is a good example of an instrument built for the purpose of studying dynamic processes. It is capable of high frame rate imaging in living human eyes of photoreceptors and of blood cells, thus enabling direct measurements of blood flow (Martin, J. A. et al. Ophthalmology 2005. 112(12): p. 2219-2224), as well as optical sectioning of the retina (Venkateswaran, K. et al. J. Biomed. Opt. 2004. 9(1): p. 132-138; Zhang, Y. et al. J. Biomed. Opt. 2006. 11, 014002). A combination of AO and ultrahigh resolution optical coherence tomography (UHR OCT) (Hermann, B. et al. Optics Letters 2004. 29(18): p. 2142-2144) has been shown to substantially increase sensitivity in comparison to standard UHR OCT and to enable unprecedented identification of intraretinal layers in the living eye with axial and transverse resolutions on the order of a few microns.
Correction, or manipulation, of the eye's aberrations with AO is central to new instruments for imaging the retina, and is critical to experiments that explore both optical and neural mechanisms in vision. MCAO has not yet been introduced but has been regarded as the next challenge in ophthalmologic AO to achieve higher resolutions over larger fields of view. Recent MCAO advancements have been made within the field of astronomy (Kelly, T. et al. Opt Express, 2000. 7: p. 368-374; Owner-Petersen, M., and Goncharov, A. J. Opt. Soc. Am. A, 2002. 19(3): p. 537-548; Knutsson, P. and Owner-Petersen, M. Opt. Express, 2003. 11: p. 2231-2237; Goncharov, A. J. et al. Opt. Express 13: p. 5580-5590).
U.S. Pat. No. 6,452,146 describes a method for the control of two phase correction devices, but it does not show the present invention using a collimator array and one camera.
U.S. Pat. No. 6,199,986 describes a method for measurement of the eye's wave aberration; however this patent does not disclose the present invention using multiple guide star sources or the present invention using a collimator array and one camera, nor does it show the present invention using a single spatial filter to simultaneously reduce unwanted light from parasitic source reflections and light from unwanted object regions from all guide stars to improve image quality in the wavefront sensor camera.
U.S. Pat. No. 6,634,750 and patent application US 2003/0038921 present an invention for a tomographic wavefront analysis system, but they do not show the present invention using a collimator array and a single spatial filter to simultaneously reduce unwanted light from parasitic source reflections and light from unwanted object regions from all guide stars to improve image quality in the wavefront sensor camera.
U.S. Pat. No. 6,736,507 presents an invention for a high resolution, multispectral, wide field of view retinal imager, but it does not show the present invention using two deformable mirrors to correct for aberrations over a wider field of view, nor does it show the present invention using a collimator array and a single spatial filter to simultaneously reduce unwanted light from parasitic source reflections and light from unwanted object regions from all guide stars to improve image quality in the wavefront sensor camera.
U.S. Pat. No. 6,964,480 presents an invention for a multiple stage phase compensator wavefront analysis system, but it does not show the present invention using a collimator array and a single spatial filter to simultaneously reduce unwanted light from parasitic source reflections and light from unwanted object regions from all guide stars to improve image quality in the wavefront sensor camera.
Goncharov et al (11 Jul. 2005/Opt. Express 13: p. 5580-5590) presents a MCAO system for telescopes using five reference sources and two deformable mirrors for multi-pupil imaging on a single detector, but they do not present the solution of our invention to use a collimator array and a single spatial filter to reduce unwanted light from parasitic source reflections and light from unwanted object regions to get sufficient image quality for human retina studies.
Normally in MCAO systems, each reference source will require its own wavefront sensor making the optical setup both bulky and expensive. The purpose of the sensor of the invention described here is both to place all Hartmann patterns on a single camera, and to clean them from spurious contributions.
Therefore, it is an object of this invention to provide a method and a product using multi-reference adaptive optics systems with spatial filtering to accurately and dynamically measure high-order aberrations in living human eyes. Aberrations change over time, with for example accommodation (the eye's lens change of focus), and with gaze angle and an individual's heartbeat. There is thus a need for real-time MCAO correction, enabling improved direct detailed studies, measurements and optical sectioning of the human retina.
Another object of this invention for correcting aberrations over a larger area in a living eye is to improve our ability to diagnose eye disease. Age-related macular degeneration is a disorder of the very center of the retina responsible for our sharpest vision, in which damage to the photoreceptor layer causes severe vision loss in the elderly. Improved retinal imaging is a key issue for early detection of this disease. Diabetes is another disease that affects the retina by the formation of microaneurysms in the retinal vasculature. Treatment requires accurate delivery of a photocoagulating laser beam, which could benefit from a higher resolution view of the critical retinal region. Glaucoma, a disease in which blindness ensues from the gradual loss of optic nerve fibers, can only be detected with conventional imaging techniques after a significant amount of damage has already occurred. More accurate measurement of the nerve fiber layer thickness around the optic nerve head will allow earlier detection of this disease.
Another object of this invention is to provide a method using multi-reference adaptive optics systems to accurately measuring high-order aberrations in individuals and customizing the correction to each person. Methods for implementing this are currently being explored: customized contact lenses, intraocular lenses, or customized laser refractive surgery.
Another object of this invention is to use the herein described methods for aberration correction and filtering of unwanted light from parasitic source reflections and light from unwanted object regions to provide improved image quality and analysis from retinal scans for biometric purposes.